My question is rather simple:
Are there experiments where they have measured lenght contractions?
I don't know for certain, but I believe that there haven't been any. In order to have a measurable length contraction, the object must be traveling near the speed of light relative to us. All experiments involving things going that fast involve subatomic particles, where length is difficult enough to define as well as to measure.
The people who design experiments for the accelerators have to take length contraction into acount. A spherical bunch of particles coming at you looks like a flattened ellipsoid due to relativistic shortening, and the detection probabilities and expected directions of ejecta are affected. So you could say that all these experiments are also testing length contraction, in that they are designed around it, and they work.
BTW, wasn't the Michelson-Morley experiment a length contraction tester? At least it was explained by Fitzgerald's length contraction formula.
The Michaelson-Morley experiment wasn't intended to, but measured length contraction as that is the explanation for the null result. Any experiment involving magnetism is also a length contraction verification as the magnetic field about a current can be explained as a length contraction effect.
As I see it, for the same reason as M&M, pretty much every experiment involving SR deals with the length contradiction at least indirectly.
So , when i get it right. When we have two refsystems, the lab and the bunch of particles. When the bunch of particles move at almost light speed, in the lab we see a ellipsoid (and in the limit at light speed a flat circular plane). But the observer in the bunch of particles sees a sphere nowheter the (relative) speed of the particles.
I'm a right?
That's exactly right.
This is actually not quite true (The same mistake wa also made by Einstein himself, so don't feel too bad...). The mistake is in the word 'look'. A length contraction can't be SEEN, it only follows from theory that there is such a thing. This all comes from another effect which needs to be taken into account. Remarkable is that this was first done by J.Terrell in 1959, while Special Relativity (SR) dates from 1905!
When a 3-D object moves with a speed near that of light, it contracts in the direction of movement as predicted by SR. But because of its extent in the direction normal to its movement, there is another effect which makes it impossible for an observer to see this contraction.
In the case of a block (http://www.lorentz.leidenuniv.nl/vanbaal/SRT/syllabus/chap6-7/img40.gif), light from the rear (as seen from the observer) takes longer to reach the observer. 'Seeing' is creating an image of all the photons that at that instant reach the retina of your eye (or in the case of a measurement the photographic surfuse e.g.). So this image consist of photons from the front of the block AND photons from the rear which departed EARLIER when the block was at an EARLIER POSITION.
So what you see is the contracted block AND one side of the block because of the effect described above. This Lorentz-contraction and apparent elongation because of the longer time light takes to reach your eye from the side of the block, results in no elongation whatsoever. Remarkable it can be proven quite easily that the combined effect is mathematically equivalent to rotating the block by an angle a (with cos(a)=1/gamma).
In the case of a sphere, rotation doesn't matter at all, so no contraction can be seen (this particular example is even more interesting because Einstein himself used the example of a fast moving sphere in one of his papers to highlight the Lorentz-contraction!)
Hope my explanation will suffice...
Ok, but what happens in the following experiment.
We have a lightsource that send out one photon and after time t another photon in the same direction. There is a photondedector at a distance l.
In the reference frame of the dedector/source the distance between the two photons is zero.
Why doesn't the dedector dedect the 2 photons at the same time?
I don't see why this would be true. I'd say that in the frame of the detector the distance between the photons is just ct, and the individual photons will be measured with the same time interval t at which they were sent.
so,why is there no lenght contraction between two photons (at a distence ct) traveling at light speed, and for a bunch of particles there is a lenght contraction?
But there is. The distance ct is the distance as measured from the frame by which the photons are moving, so it is the length-contracted distance. From the Photons' point of view the distance is infinite. (Mainly because, due to time dilation, from the photons' perspective the time between the emission of each photon is infinite.)
And the particles in the bunch are traveling at the same speed so the whole bunch is in the same frame, and gets Lorentz boosted as a whole relative to the detectors.
In some Quantum setups, there are similarities between your question, and Paramtric Downconversion: http://info.fysik.dtu.dk/optics/highlights/intro-1.htm
Peterdevis, I do not understand the experiment or your explanation of it. Is the photodetector at rest with respect to the light source?
If so, in the rest frame of the photodetector, the first photon takes l/c (length/velocity) to arrive at the photodector. The second arrives at the time t + 1/c, and, at the time of its emission is behind the first by ct, if the time of the first photon emission is zero.
Janus, the proper time of each photon is always zero. If you do know that, then I do not understand your last post.
I hope these comments help to clarify things a bit.
Oh stupid me, ofcourse we can only measure the length-contracted distance in our ref system. Thanks!
Yes, the photodedector is at rest in respect to the light source. I thought that the distance in the photonframe equals ct, but ct is already the length-contracted distance in the dedector frame.
Is this length contraction a physical phenomen or is it just optical, because nothing is happening with the space between the two photons.
Or can't we make no distinction between "measering" and "reality"
I don't think it helps to think that a photon has a rest frame. The mathematics won't work anyway for that case.
Special Rel. describes what each observer sees or measures. It is not just optical. It could be with sound waves. Or maybe even odors (smell). Let's just say it is physical.
Is reality real? What is reality? How do we know what is there for us to know about reality? Such questions are best answered in the philosophical forums.
The role of measuring in Quantum Mechanics, which is a relevant topic to your question about the distinction between measurement and reality, is still controversal. Maybe you should go to the QM forum as well.
I am sorry I couldn't be any more help, but far greater minds than mine have wrestled with such questions for centuries now, and this seems to be the best that I can offer you from philosophy.
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